protobuf_generate and FileDescriptor

[mjheilmann]

We have code to compensate for the service definitions of protobuf_generate being a FileDescriptorProto instead of a ServiceDescriptorProto. But we handle it by simply extracting the ServiceDescriptorProto and passing that into the original builder code.

When we are presented with a FileDescriptorProto, we should use it directly as a reflection payload instead of ignoring the FileDescriptorProto and simulating our own. ref